1. Field of the Invention
This invention relates to switching valves, and more particularly to an improved switching valve with pressure compensation for a fuel injector with a pressure booster.
2. Description of the Prior Art
For introducing fuel into the combustion chambers of direct-injection internal combustion engines, it is known to use stroke-controlled injection systems with a high-pressure storage chamber (common rail). The advantage of these injection systems is that the pressure of fuel injected into the combustion chamber can be adapted to the load and the rpm of the engine over wide ranges. For reducing emissions and attaining high specific performance, a high injection pressure is necessary. The attainable pressure level of high-pressure fuel pumps is limited for reasons of strength, so that for further increasing the pressure in fuel injection systems, pressure boosters in the fuel injectors are employed.
The subject of German Patent Disclosure DE 101 23 913 A1 is a fuel injection system for internal combustion engines with a fuel injector that can be supplied from a high-pressure fuel source. Between the fuel injector and the high-pressure fuel source, there is a pressure booster system that has a movable pressure booster piston. The pressure booster piston separates a chamber that can be connected to the high-pressure fuel source from a high-pressure chamber that communicates with the fuel injector. By filling a differential pressure chamber of the pressure booster system with fuel, or evacuating fuel from the differential pressure chamber, the fuel pressure in the high-pressure chamber can be varied. The fuel injector has a movable closing piston for opening and closing injection openings. The closing piston protrudes into a closing pressure chamber, so that the closing piston can be acted upon by fuel pressure in order to attain a force acting on the closing piston in the closing direction. The closing pressure chamber and the differential pressure chamber are formed by one common closing pressure differential pressure chamber, and all the partial regions of the closing pressure differential pressure chamber communicate with one another permanently for exchanging fuel. A pressure chamber for supplying fuel to the injection openings and for subjecting the closing piston to a force acting in the opening direction is provided. A high-pressure chamber communicates with the high-pressure fuel source in such a way that aside from pressure fluctuations, at least the fuel pressure of the high-pressure fuel source can prevail constantly in the high-pressure chamber; the pressure chamber and the high-pressure chamber are formed by a common injection chamber. All the partial regions of the injection chamber communicate with one another permanently for exchanging fuel.
German Patent Disclosure DE 102 294 15.1 relates to a device for damping the needle stroke in pressure-controlled fuel injectors. A device for injecting fuel into a combustion chamber of an internal combustion engine is disclosed which includes a fuel injector that can be subjected to fuel that is at high pressure via a high-pressure source. The fuel injector is actuated via a metering valve; an injection valve member is surrounded by a pressure chamber, and the injection valve member can be urged in the closing direction by a closing force. The injection valve member is assigned a damping element, which is movable independently of it and which defines a damping chamber and has at least one overflow conduit for connecting the damping chamber to a further hydraulic chamber. According to DE 102 294 15.1, the control of the fuel injector is effected with a 3/2-way valve, as a result of which an economical injector that is economical in terms of installation space can indeed be made, but this valve must control a relatively large return quantity from the pressure booster.
Instead of the embodiment of a 3/2-way valve, known from DE 102 294 15.1, servo valves may also be employed, which are embodied as leakage-free in the state of repose of the servo valve at the guide portion, which favorably affects the efficiency of a fuel injector. However, the fact that in the open state of the servo valve piston of the 3/2-way valve, no pressure face pointing in the opening direction of the valve is subjected to system pressure is a disadvantage. Moreover, a slow opening speed of the servo valve piston cannot be attained, which means that the least-quantity capability of a servo valve configured in this way is limited. In the open state of the servo valve piston, only an inadequate closing force ensues at a second valve seat embodied on it, and this can lead to leaks and increased wear.
In the servo valves known from the prior art, the major complexity in terms of production on the one hand and the attendant costs on the other are disadvantages.